1. Field of the Invention
This invention relates generally to apparatus and methods for producing heat exchange with body tissue, and more specifically to methods and apparatus for the hypothermic treatment of a body fluid in a body conduit.
2. Discussion of the Prior Art
Many of the advantages of hypothermia are well known. By way of example, it has been found particularly desirable to lower the temperature of body tissue in order to reduce the metabolism of the body. In stroke and several other pathological conditions, hypothermia also reduces the permeability of the blood/brain barrier. It inhibits release of damaging neurotransmitters and also inhibits calcium-mediated effects. Hypothermia inhibits brain edema and lowers intracranial pressure.
Some of the disadvantages of systemic hypothermia include cardiac arrhythmia, pulmonary edema and coagulopathies. Systemic hypothermia also results in hypotension and various immunodeficiencies.
In the past, hypothermic treatment has been addressed systemically, meaning that the overall temperature of the entire body has been lowered to achieve the advantages noted above. This has been particularly desirable in surgical applications where the reduced metabolism has made it possible to more easily accommodate lengthy operative procedures. An example of this systemic approach includes catheters for transferring heat to or from blood flowing within a patient""s vessel, as disclosed by Ginsburg in U.S. Pat. No. 5,486,208. A closed loop heat exchange catheter is also disclosed by Saab in U.S. Pat. No. 5,624,392.
The systemic approach is not always advantageous when the beneficial effects are desired locally at the focus of the operative procedure and only the disadvantages of hypothermia are felt throughout the remainder of the body.
As a result, more recent focus has been directed to producing hypothermia in localized areas of the body, leaving the remainder of the body to function at a normal body temperature. These localized applications of hypothermia have been external, relying for example on cooling helmets or cooling neck collars to produce localized hypothermia for the brain.
A heat exchange catheter and method of operation are included in the present invention. The method is adapted to produce hypothermia or hyperthermia in a selected portion of the body without substantially varying the temperature of the remaining portions of the body. The selected body portion will typically be associated with a body conduit which conveys a body fluid to the selected body portion. Of particular interest are the organs of the body which are commonly nourished and maintained by a flow of blood in the arterial system. For example, a flow of blood is introduced to the brain through the carotid artery. Of course the temperature of this blood is usually at the normal body temperature.
By positioning a heat exchange catheter in the body conduit, heat can be added to or removed from the body fluid to heat or cool the selected body portion. For example, the heat exchange catheter can be disposed in the carotid artery where the arterial blood flowing to the brain can be cooled. The flow of cooled blood to the brain reduces the temperature of the brain thereby resulting in cerebral hypothermia. Importantly, this temperature reduction occurs primarily and selectively in the brain; the remaining portions of the body maintain a generally normal body temperature. In accordance with this method, the selected body portion, such as the brain, can be cooled thereby providing the advantages associated with hypothermia for this body portion. The remainder of the body, such as the portions other than the brain, do not experience the reduction in temperature and therefore are not susceptible to the disadvantages of hypothermia. Furthermore, the invention is intended to remotely alter temperature in a region other than the point of introduction into the body. this is different than devices intended for systemic temperature control.
Several factors are of interest in effecting heat transfer in a heat exchanger. These factors include, for example, the convection heat transfer coefficient of the two fluids involved in the heat exchange, as well as the thermal conductivity and thickness of the barrier between the two fluids. Other factors include the relative temperature differential between the fluids, as well as the contact area and residence time of heat transfer. The Reynolds number for each fluid stream affects boundary layers, turbulence and laminar flow.
With concern for these factors, the heat exchange catheter of the present invention includes a shaft having an axis, a fluid inlet lumen and a fluid outlet lumen each extending generally between a proximal end and a distal end of the shaft. A hub disposed at the proximal end provides access to the fluid lumens. At least one balloon is provided in a heat exchange region at the distal end of the shaft, the balloon wall providing the barrier between the two fluids. With the catheter positioned in contact with the body fluid within the conduit, heat transfer occurs across the balloon wall. The relative temperature differential is facilitated with countercurrent flow between the two fluids.
In one aspect of the invention, a first balloon is disposed at the distal end of the shaft and defines with the shaft an inflatable first cavity. Portions of the shaft define a first inlet hole extending in fluid communication between the first lumen and the first cavity. Portions of the shaft define a first outlet hole extending in fluid communication between the first cavity and the fluid outlet lumen. A second balloon disposed relative to the first balloon defines with the shaft an inflatable second cavity with portions of the shaft defining a second inlet hole between the fluid inlet lumen and the second cavity. Portions of the shaft also define a second outlet hole in fluid communication with the second cavity and the fluid outlet lumen. Typically, the first balloon will be disposed distally of the second balloon and the first inlet hole will be larger than the second inlet hole. An elastomeric material covering a valley or volume between the first balloon and the second balloon may be provided to promote mixing necessary for efficient heat exchange yet minimize turbulence and shear which can be damaging to blood.
In an additional aspect of the invention, a method for exchanging heat with a body fluid in a body conduit includes the step of introducing into the body conduit a catheter having an inlet lumen and an outlet lumen. The catheter is provided with a first cavity and a second cavity each in heat transfer relationship with the body fluid in the body conduit. A heat exchange fluid is introduced into the inlet lumen and through an inlet hole into each of the first cavity and the second cavity. An exchange of heat then occurs between the heat exchange fluid in the first and second cavities and the body fluid in the body conduit. Ultimately, the heat exchange fluid is removed through an outlet hole and the outlet lumen associated with each of the first cavity and the second cavity. Creating non laminar flow in one or both of the heat exchange fluid and the body fluid will improve heat transfer efficiency. Heat transfer can also be effected by various structures which either enhance or inhibit turbulence in the fluids.
These and other features and advantages of the invention will be better understood with a description of the preferred embodiments of the invention and reference to the associated drawings.